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Abstract:

A method of deterring mammals comprising configuring an acoustic
deterrent system to emit an acoustic signal having characteristics which
repel the mammals by eliciting an acoustic startle response reflex in
said mammals.

Claims:

1. A method of deterring mammals comprising configuring an acoustic
deterrent system to emit an acoustic signal having characteristics which
cause said mammals to be repelled from an area, by eliciting an acoustic
startle response reflex in said mammals.

2. A method as claimed in claim 1 wherein the acoustic signal has
characteristics which sensitize said mammals such that repeated exposure
to the signal reinforces the acoustic startle response reflex therefore
increasing aversive responses in the mammals over time.

3. A method as claimed in claim 2 wherein said acoustic signal comprises
isolated acoustic pulses, the method comprising repeatedly exposing the
mammals to said isolated acoustic pulses so as to sensitize said mammals.

4. A method as claimed in claim 3 wherein each interval between isolated
pulses is longer than 500 ms.

5. A method as claimed in claim 3 wherein said pulses each have a short
duration, and the acoustic signal has a low duty cycle, such that in use
the energy consumption of the system is low.

6. A method as claimed in claim 5 wherein the duration of each pulse is
less than 1 second and the duty cycle of the acoustic signal is less than
10%.

7. A method as claimed in claim 1 wherein the acoustic signal comprises
single, isolated pulses emitted at random intervals, each having a short
rise time.

8. A method as claimed in claim 1 wherein said characteristics that repel
the mammals by eliciting an acoustic startle response reflex comprise a
high amplitude and short rise time.

9. A method as claimed in claim 8 wherein the amplitude of the acoustic
signal exceeds targeted mammals' auditory threshold by more than 80 dB
within 100 ms of the signal's onset.

10. A method as claimed in claim 8 wherein the amplitude of the acoustic
signal exceeds targeted mammals' auditory threshold by more than 80 dB
within 50 ms of the signal's onset.

11. A method as claimed in claim 1 wherein signal bandwidth spans at
least one third of an octave.

12. A method as claimed in claim 1 wherein said acoustic signal is
emitted when the device is triggered by a sonar system detecting the
presence of an animal.

13. A method as claimed in claim 1, wherein incurring an acoustic startle
response comprises the steps of: selecting a target order, family, genus
or species of mammals to be deterred; selecting a received level at a
predetermined level above a representative hearing threshold of the
targeted mammals; transmitting an acoustic signal from a transmission
point at a source level required, taking into account transmission loss,
to create the selected received level at a predetermined distance from
the transmission point.

14. A method as claimed in claim 13, wherein the predetermined level is
between 80 and 130 dB above the representative hearing threshold at each
frequency.

15. A method as claimed in claim 13, wherein the acoustic signal has a
duration about as long as the acoustic integration time specific to the
targeted mammals auditory system.

16. A method as claimed in claim 13, wherein the acoustic signal has a
duration of less than 200 ms.

17. A method as claimed in claim 13, wherein the acoustic signal has a
rise time of less than 20 ms.

18. A method as claimed in claim 13, wherein the acoustic signal is as
broadband as possible within the designated frequency range.

19. A method as claimed in claim 13, wherein the acoustic signal
comprises frequency components at which the aural sensitivity of the
targeted mammals is greater than that of selected other animals.

20. A method as claimed claim 13, wherein the acoustic signal uses a
frequency greater than 500 Hz for the deterrence of seals or other
pinnipeds.

21. A method as claimed in claim 13, wherein the mammals to be deterred
comprise sea mammals including pinnipeds, most preferably seals; or
cetaceans.

22. A method as claimed in claim 13, wherein the mammals to be deterred
comprise land mammals.

23. An acoustic deterrent device comprising a method as claimed in claim
1, wherein the acoustic deterrent system comprises a signal transducer
arranged to transmit said acoustic signals in accordance with the method
of claim 1 signal.

24. An acoustic deterrent system comprising a method as claimed in claim
1, wherein the acoustic deterrent system comprises a control unit, a
power source, amplifier and transducer means, co-operable to perform the
method of claim 1 arranged to produce said acoustic signal.

25. A method as claimed in claim 24, wherein the acoustic deterrent
system includes a control unit that operates under the control of control
soft are executable on a computer so that the computer is operable as the
control unit of claim 24.

26. A method as claimed in claim 25, wherein the control software is
stored on a computer readable medium comprising the control software as
claimed in claim 25.

27. A method of deterring mammals comprising configuring an acoustic
deterrent system to emit an acoustic signal, said acoustic signal having
characteristics which cause said mammals to be repelled by eliciting an
acoustic startle response reflex in said mammals, wherein the acoustic
signal comprises single, isolated pulses at random intervals, the
amplitude of the acoustic signal exceeding the targeted mammals' auditory
threshold by more than 80 dB within 100 ms of the signal's onset.

28. A method as claimed in claim 27 wherein the acoustic signal has
characteristics which sensitize said mammals such that repeated exposure
to the signal reinforces the acoustic startle response reflex, therefore
increasing aversive responses in the mammals over time.

29. A method as claimed in claim 28 wherein said acoustic signal
comprises isolated acoustic pulses, the method comprising repeatedly
exposing the mammals to said isolated acoustic pulses so as to sensitize
said mammals.

30. A method as claimed in claim 29 wherein each interval between
isolated pulses is longer than 500 ms.

31. A method as claimed in claim 28 wherein the bandwidth of the signal
spans at least one third of an octave.

32. A method as claimed in claim 29 wherein said pulses each have a short
duration, and the acoustic signal has a low duty cycle, such that in use
the energy consumption of the system is low.

33. A method as claimed in claim 32 wherein the duration of each pulse is
less than 1 second and the duty cycle of the acoustic signal is less than
10%.

34. A method as claimed in claim 28 wherein incurring an acoustic startle
response comprises the steps of: selecting a target order, family, genus,
or species of mammals to be deterred, selecting a received level at a
predetermined level above a representative hearing threshold of the
targeted mammals, and transmitting an acoustic signal from a transmission
point at a source level required, taking into account transmission loss,
to create the selected received level at a predetermined distance from
the transmission point.

35. An acoustic deterrent system for deterring mammals, comprising a
control unit, a power source, an amplifier, and a transducer arranged to
emit an acoustic signal, said acoustic signal having characteristics
which cause said mammals to be repelled from an area, by eliciting an
acoustic startle response reflex in said mammals.

36. An acoustic deterrent system as claimed in claim 35 wherein the
amplitude of the acoustic signal exceeds targeted mammals' auditory
threshold by more than 80 dB within 100 ms of the signal's onset.

37. An acoustic deterrent system as claimed in claim 35 wherein the
amplitude of the acoustic signal exceeds targeted mammals' auditory
threshold by more than 80 dB within 50 ms of the signal's onset.

38. An acoustic deterrent system as claimed in claim 35 wherein signal
bandwidth spans at least one third of an octave.

39. An acoustic deterrent system as claimed in claim 35 wherein the
acoustic signal has characteristics which sensitize said mammals such
that repeated exposure to the signal reinforces the acoustic startle
response reflex therefore increasing aversive responses in the mammals
over time.

40. An acoustic deterrent system as claimed in claim 39 wherein said
acoustic signal comprises isolated acoustic pulses, said acoustic
deterrent system being arranged to repeatedly expose the mammals to said
isolated acoustic pulses so as to sensitize said mammals.

41. An acoustic deterrent system as claimed in claim 40 wherein each
interval between isolated pulses is longer than 500 ms.

42. An acoustic deterrent system as claimed in claim 40 wherein said
pulses each have a short duration, and the acoustic signal has a low duty
cycle, such that in use the energy consumption of the system is low.

43. An acoustic deterrent system as claimed in claim 42 wherein the
duration of each pulse is less than 1 second and the duty cycle of the
acoustic signal is less than 10%.

44. An acoustic deterrent system as claimed in claim 35 wherein the
acoustic signal comprises single, isolated pulses emitted at random
intervals, each having a short rise time.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of application Ser. No.
12/596,337, filed Jan. 21, 2010, and is assigned to the same assignee as
the prior application.

BACKGROUND

[0002] This application relates to acoustic deterrence and, in particular,
to an acoustic deterrent device and methods for deterring mammals, most
particularly but not exclusively marine mammals.

[0003] Worldwide, farming of marine and diadromous finfish species has
experienced tremendous growth rates, showing a ten fold increase over the
last three decades. This increase in potential food resources presented
in a marine environment has brought about increased interactions with
predatory species. One common group of predators is marine mammals who
exploit food resources depending on their profitability and potential
costs, which include dive depths as a major factor. The shallow depth of
fish farms thus makes them particularly attractive to predators.

[0004] In particular, predatory behaviour of pinnipeds is a major concern,
causing a variety of economical and market related risks for the fish
farm owner. Accordingly, there is much interest in developing
anti-predator control methods.

[0005] These methods include net modifications, lethal or non-lethal
removals, population control and aversive conditioning. However, each of
these methods has their own drawbacks. For example, the addition of a
second net can cause tangling of predators and non-predatory species; and
lethal removals as well as population control may have an impact on
populations and raise ethical concerns over the treatment of the animals.
Culling of higher order predators can also have negative impact on
predation rates by other predators, for example, pinnipeds forage on
predatory fish species around the net pen which in turn potentially feed
on aquaculturally important species. Also, emetic aversion conditioning
requires that individuals learn to associate treated fish with sickness,
and this can be hard to achieve when predator numbers are high.

[0006] One anti-predator control method that avoids these pitfalls is the
use of acoustic deterrent devices (ADDs). These have traditionally been
considered to be a benign solution. However, they do present certain
problems with respect to the effects they have on other marine wildlife
and with habituation, where a target species motivated by a food source
ceases to be deterred by the acoustic signals.

[0007] If the source level of an ADD is sufficiently high it can cause
temporary or permanent hearing damage both to the targeted species and to
other wildlife, and the noise pollution is in general an environmental
hazard. Further, both targeted and non-targeted species can be excluded
from their natural habitat within a wide radius of the fish farms. These
concerns have led some governments to restrict or even ban the use of
ADDs.

[0008] A further problem is the habituation of the target species to the
sound. In extreme cases, the sound which is intended to be aversive acts
as a "dinner bell" and actually serves to attract predators, rather than
deter them. Also, if predator sounds are used as an aversive noise,
habituation is dangerous for the target species once they had returned to
their normal habitat.

[0009] Also, existing power levels and signal cycles impose heavy duty
cycles on the batteries used as power sources in the transducer units.

[0010] Accordingly, it would be desirable to provide an acoustic deterrent
device that is highly effective, but does not damage the environment, is
species specific and avoids habituation.

[0011] According to a first aspect of the present invention there is
provided a method of deterring mammals comprising configuring an acoustic
deterrent system to emit an acoustic signal having characteristics which
repel the mammals by eliciting an acoustic startle response reflex in
said mammals.

[0012] "Deterring" is taken to mean discouraging or preventing a mammal
from entering into or staying in a particular area. The startle reflex
response is a physiological reflex in mammals to particular sounds, which
often initiates flight from the sound, thereby repelling mammals from a
location in the vicinity of the sound. The startle reflex response should
not be confused with an animal or person being "started" or "startled" in
the colloquial or everyday sense.

[0013] In an embodiment, the method of incurring an acoustic startle
reflex response comprises the steps of: [0014] selecting a target
order, family, genus or species of mammals to be deterred; [0015]
selecting a received level at a predetermined level above a
representative hearing threshold of the targeted mammals; [0016]
transmitting an acoustic signal from a transmission point at a source
level required, taking into account transmission loss, to create the
selected received level at a predetermined distance from the transmission
point.

[0017] In an embodiment, the predetermined level is between 90 dB and 125
dB re 1 μPa above the representative hearing threshold at each
frequency.

[0018] In an embodiment, the acoustic signal has a duration about as long
as the acoustic integration time specific to the targeted mammal's
auditory system

[0019] In an embodiment, the acoustic signal has a duration of less than
200 ms.

[0020] In an embodiment the acoustic signal has a rise time of less than
20 ms.

[0021] In an embodiment, the acoustic signal is as broadband as possible
within the designated frequency range.

[0022] In an embodiment, the acoustic signal comprises frequency
components at which the aural sensitivity of the targeted mammals is
greater than that of selected other animals.

[0023] In an embodiment, the acoustic signal uses a frequency band between
500 Hz and 2 kHz for the deterrence of seals or other pinnipeds. The
acoustic signal is as broadband as possible within this frequency range.

[0024] In an embodiment, the mammals to be deterred comprise sea mammals
including pinnipeds, such as seals; or cetaceans.

[0025] Alternatively, the mammals to be deterred comprise land mammals.

[0026] In an embodiment, a secondary conditioning sound is made to
condition the mammals to avoid the main stimulus. This could be a sound
of centre frequency 1 kHz, modulation rate 250 Hz and duration 1.2
seconds with two modulation cycles.

[0027] In an embodiment, the conditioning sound is played between 500 ms
and 5 s before the main acoustic signals on selected occasions.

[0028] According to a second aspect of the invention, there is provided a
method of deterring marine mammals comprising transmitting an acoustic
signal which comprises an aversive sound whose characteristics are chosen
based on characteristics that are unpleasant to humans.

[0029] In an embodiment, the aversive sound is selected to have one or
more of the following psychophysical features: high roughness; low
tonality; high loudness; high sharpness.

[0030] In an embodiment a frequency modulation is applied to the carrier
signal, most preferably with a modulation frequency of about 70 Hz.

[0031] This frequency modulation is applied in order to achieve high
roughness.

[0032] In an embodiment, the frequency modulation has a modulation depth
of between 10 and 150%, preferably 50% of the centre frequency of the
carrier signal.

[0033] In an embodiment, the aversive sound has a sound pressure level of
at least 70 dB above the hearing threshold of the targeted mammals, most
in an embodiment at least 80 dB above the hearing threshold of the
targeted mammals.

[0034] In an embodiment, an aversive sound is of a complex form and
comprises partials that fall within 25% of the critical bandwidth of the
hearing system of the targeted mammals.

[0035] In an embodiment, when cetaceans are to be deterred the acoustic
signals could be close to the upper frequency edge of the hearing range
in order to increase sharpness.

[0036] In an embodiment, the acoustic signal is as broadband as possible
within the selected frequency band in order to increase loudness without
increasing the actual sound pressure level.

[0037] In an embodiment, the aversive sound comprises frequency components
at which the aural sensitivity of the targeted mammals is greater than
that of selected other animals.

[0038] In an embodiment, the acoustic signal comprises one or more
frequency components between 500 Hz and 2 kHz for the deterrence of seals
or other pinnipeds.

[0039] In an embodiment, sound exposure time is determined based on a
sound exposure level below that which causes a temporary threshold shift
in the target and non-target species, for example an energy flux density
of 120 dB re 1 μPa2s-1 above the hearing threshold of the
targeted mammals.

[0040] According to a third aspect of the present invention there is
provided an acoustic deterrent device comprising a signal transducer
arranged to transmit acoustic signals in accordance with the method of
the first aspect and/or the method of the second aspect.

[0041] According to a fourth aspect of the present invention there is
provided an acoustic deterrent system comprising a control unit, a power
source, amplifier and transducer means, co-operable to perform the method
of the first aspect and/or the method of the second aspect.

[0042] According to a fifth aspect of the invention there is provided
control software executable on a computer so that the computer is
operable as the control unit of the fourth aspect.

[0043] The control software can be provided recorded on a computer
readable medium, or made available for download.

[0044] According to a sixth aspect of the present invention there is
provided an acoustic signal which incurs a startle reflex response in
mammals in order to deter them. The acoustic signal can be produced and
used in accordance with any of the previous mentioned aspects.

[0045] According to a seventh aspect of the invention there is provided an
acoustic signal for the deterrence of mammals which comprises an aversive
sound whose characteristics are chosen based on characteristics that are
unpleasant to humans. The acoustic signal can be produced and used in
accordance with any of the previous mentioned aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The present invention will now be described, by way of example
only, with reference to the accompanying figures in which:

[0051] FIG. 5 shows a comparison of seal sightings during field trials on
both control days and sound days for (a) closest approaches and (b)
average distance from an Acoustic Deterrent Device;

[0052] FIG. 6 shows a comparison of porpoise sightings during field trials
on both control days and sound days for (a) closest approaches and (b)
average distance from an Acoustic Deterrent Device;

[0053] FIG. 7: shows a graph of results from experiments with captive
seals showing the seals exhibited a startle reflex response and
sensitised to a transmitted sound meaning that aversive responses
increased over time.

DESCRIPTION OF THE EMBODIMENTS

[0054] A variety of Acoustic Deterrent Devices (ADDs) are available to
reduce or stop predation of pinnipeds on finfish farms. These include for
example the Ferranti-Thomson Mk2, Mk3 and 4× Seal scrammers, the
Ace-Aquatec "silent scrammer", the Airmar Technology Corporation dB Plus
II, the Terecos Limited type DSMS-4 and the Lofitech "universal scarer"
or "seal scarer".

[0055] As seen in FIG. 1, an ADD comprises a power source 10 (usually
marine batteries), a control unit 12, an amplifier 14 and an underwater
transducer (speaker) 16. The embodiment shown in FIG. 1 shows all of the
power source, amplifier and transducer being below the water surface 18,
but it will be appreciated that any suitable arrangement of these
components can be used, for example one or more of the power source and
amplifier may be situated remote from the transducer 16 and as such could
be above the surface 18, or as a further example, all the components
could be underwater, not just the transducer.

[0056] The control unit 12 typically includes a computer that has a number
of sound files stored on it which generate signals to be relayed through
the amplifier 14 and broadcasted into the water. The control unit 12 also
controls the timing of the sounds which are played.

[0057] Sound being played is characterised by its source levels, rise
time, frequency composition and duration. Additionally, the inter-sound
interval determines how quickly sounds follow each other.

[0058] The "source level" (SL) is a measurement of the acoustic output of
the device at 1 m distance. In the following text, source levels and
received levels in general will be denoted in units of decibels (dB)
measured with reference to 1 μPa, unless a specific statement or
context implies otherwise. The "rise time" is a measure of how long it
takes for an acoustic signal or pulse to reach its maximum amplitude. The
term "sensation levels" refers to the sound pressure level by which a
stimulus exceeds the species' auditory threshold (received level minus
hearing threshold). Received level refers to the sound pressure level
that reaches the animal's ears (source level minus transmission loss).
The term sound exposure level (SEL) refers to the energy flux density
(being a function of sound pressure level and exposure time) and is given
by SEL=SPL+10 log10 (exposure time) where SPL is the sound pressure
level of a received sound.

[0059] When designing an acoustic deterrent device there are various
factors that must be taken into account, including ecological impacts (on
both target and non-target species), and problems and potential
solutions.

Ecological Impacts

Species of Concern

[0060] Any animal that can perceive acoustic sounds can be potentially
adversely affected by them. These affects can be wide ranging. For
example, the Ferranti-Thomson 4×ADD has a power of over 200 dB re 1
μPa at 25 kHz and the signals from this device can be audible to a
harbour porpoise (Phocoena phocoena) for up to 10 km. The zone of
potential audibility for a harbour seal for a 175 dB re 1 μPa source
is thought to lie between approximately 1.4 km and 2.9 km.

Hearing Damage

[0061] ADDs could cause hearing damage to target species and to non-target
species, which leads to adverse effects on individual animals and the
population in general. Hearing damage would also reduce the potential
efficiency of the ADD as it would become less audible to the affected
predators.

[0062] Hearing damage first occurs as a temporary shift of the hearing
threshold (TTS) that is fully recoverable after a few hours or days.
However, exposure to higher intensity or longer duration acoustic stimuli
can cause chronic damage and lead to a permanent threshold shift (PTS).
In its mildest form this permanent hearing damage only affects the outer
hair cells of the auditory system. This leads to a very subtle rise of
the hearing threshold, but also destroys the cochlea amplifier causing a
diminishing of the dynamic range and a loss of the ability to
discriminate between frequencies.

[0063] Hearing damage in any form is a function of sound pressure level
(SPL) and exposure time. A sound with a short duration can be safely
presented at a higher SPL than a longer one. It has been suggested that
stimuli of equal acoustic energy cause similar damage. The sound exposure
level (SEL) or energy flux density has been suggested as a measure for
defining safe exposure levels, where SEL=SPL+10 log10 (exposure
time). However, data on terrestrial mammals seems to suggest that the
equal energy criterion underestimates the risk of hearing damage, at
least for sound pressures close to a critical level of about 135 dB above
the hearing threshold.

[0064] No direct measurements of PTS are available for marine mammals, so
conclusions have to be drawn based on extrapolation from TTS data or
human damage risk criteria (DRC).

Temporary Threshold Shift (TTS)

[0065] Studies on odontocetes have found that sound exposure levels
between 193 dB and 213 dB re 1 μPa2s-1 can cause mild to
moderate, but fully recoverable TTS. These values are about 100 dB-130 dB
re 1 μPa above the hearing threshold of the tested individuals (sound
exposure level-sensation level).

[0066] Studies on odontocetes have been used to estimate TTS ranges of
ADDs for single transmissions (i.e. short pulses) based on equal energy
assumptions. Given these assumptions an Airmar dB Plus 11 device (having
a source level of 192 dB re 1 μPa) would only cause TTS in bottlenose
dolphins at distances closer than 1 m while a high power (200 dB re 1
μPa) Ferranti-Thomson 4× device would have a TTS zone of about
2-3 meters. TTS zones for the harbour porpoise would be 2-3 and 14-25
meters respectively.

[0068] Human damage risk criteria (DRC) states that PTS will be caused at
or after a critical value of 130 dB above the hearing threshold. Studies
of terrestrial mammals have confirmed that such hearing damage occurs
quickly when exposed to sound pulses at 130 dB-140 dB above the hearing
threshold. Available data on harbour porpoises suggests that a PTS damage
zone for harbour porpoises would be 30 m, with a similar result for
killer whales (Orcinus orca).

[0069] Extrapolation of thresholds or PTS from TTS data is problematic,
but due to a lack of direct measurements in marine mammals and the
difficulties of extrapolation from human DRC such an attempt is
justified. Data on humans suggests that exposure levels causing TTS of 40
dB or more carry some risk of causing a PTS. A temporary threshold shift
that exceeds 40 dB carries some risk to become permanent and correlates
with an increase of the sound exposure pressure level by 20 dB beyond the
sound pressure level that causes onset TTS). When applying a sound
exposure sensation level criterion of 115 dB re (hearing threshold in
Pa)2-s and adding respective auditory thresholds then a 10 s
emission from a seal scarer with a source level of 193 dB re 1 μPa
would the following damage zones: 18 m, 35 m and 79 m for the bottlenose
dolphin, harbour porpoise, and killer whale respectively

[0070] Long term exposure over months or years requires even more
conservative criteria. Accepted noise levels at human industrial
workplaces are 85 dB above the hearing threshold zone. An even more
conservative 80 dB threshold would be exceeded within a zone of over a
kilometre radius for the Airmar dB Plus II device which has a source
level of 192 dB re 1 μPa. In areas with dense fish farming activity,
animals could be exposed to these levels for extensive amount of time. As
studies on humans have shown, initially harmless TTS can turn into PTS if
recovery periods are insufficient or non-existent.

[0071] Hearing in fish is less well studied in general. However, fish are
sensitive to lower frequencies than pinnipeds or cetaceans and studies on
fish have been carried out using signals with frequencies of 500 Hz or
less, which is within the most sensitive hearing range of fish. This
makes it difficult to draw conclusions about the effects of higher
frequency signals. However, increasing TTS with increasing exposure
levels and weak temporary shifts have been demonstrated in some studies.

Masking

[0072] It is important that the sounds produced by ADDs do not overlap
with communication or echolocation sounds used by target or non-target
mammals.

[0073] For a signal to be masked the detection of the signal must be
influenced by a second sound--the masker, which will usually be centred
at the frequency of the signal. It has been well established that the
masking effect is dependent on the bandwidth of the masker until it
reaches a so-called critical bandwidth. Therefore, noise only masks a
signal if it contains similar frequencies to the signal of interest.
Critical bandwidths in marine mammals are generally below 10% of the
signal centre frequency.

[0074] Additionally, masking effects are attenuated if the masker and the
signal come from different directions. In harbour seals minimum
distinguishable audible angles for clicks are 4.5 degrees, and in
bottlenose dolphins they are less than 3 degrees. Therefore, it seems
that cetaceans and pinnipeds may successfully avoid masking effects, but
the potential to affect other marine mammal communication networks is
high.

[0075] Little is known about the impacts of masking on fish. However,
their hearing abilities are generally less sophisticated than those of
mammals which could make them more prone to masking effects.

Habitat Exclusion

[0076] As mentioned above, ADDs for seals have been shown to exclude
non-target marine mammals (i.e. cetaceans) from their habitat. This has
been confirmed by several studies.

PROBLEMS AND SOLUTIONS

Duty Cycles

[0077] If an existing ADD is used continuously, noise pollution is
substantial. Duty cycles range from 3% in a Ferranti-Thomson model up to
50% in other designs.

[0078] Devices can include additional predator detectors so that the ADD
is only triggered when a predator is present. This can be via direct
detection of a predator, or from the analysis of the motion patterns of
fish in the pens of the fish farm. Such systems are desirable and can be
incorporated in combination with the invention.

Frequency Bands

[0079] FIG. 2 shows the hearing thresholds for a spectrum of marine
wildlife measured in dB with respect to 1 μPa plotted on the Y axis,
against frequency in kHz on the logarithmic X axis.

[0080] Existing ADDs use frequencies above 4 kHz, at which odontocetes'
hearing is generally more sensitive than pinnipeds' hearing. Thus
odontocetes perceive a sound of a given SPL as louder than seals do.

[0081] Discomfort levels for a captive harbour seals and harbour porpoises
have been investigated and it has been found that for a frequency of 12
kHz, harbour porpoises avoid sound that is approximately 5 dB quieter
than that avoided by harbour seals. This corresponds to the difference
between the hearing thresholds of both species at the relevant frequency.

[0082] Some ADDs operate at frequencies close to the most sensitive
hearing of pinnipeds, which is between 20 kHz and 30 kHz. However, these
frequencies are not suitable because hearing thresholds in odontocetes
are even lower in this band. Furthermore, most odontocetes have their
most sensitive hearing in the ultrasonic range between 30 and 50 kHz. It
would therefore be desirable that no ADD should produce substantial
energy above 20 kHz. However, this is the case for the majority of
available ADDs.

[0083] In a frequency band between 200 Hz and 2 kHz pinnipeds' hearing is
more sensitive than odontocetes' hearing and thus appears to be an ideal
frequency band for ADDs. This frequency band has not been previously used
for pinniped ADDs because it is below the frequencies at which their
hearing is most sensitive. However, the inventors have realised that the
use of this apparently non-optimal frequency band enables an ADD to have
an effect on pinnipeds without adversely affecting cetaceans and
odontocetes.

[0084] This is a specific example of the inventors' more general
realisation that the frequency components for an ADD can be chosen to lie
at values at which the aural sensitivity of the targeted mammals is
greater than that of selected other animals, even at the expense of using
the frequencies corresponding to the lowest hearing thresholds of the
targeted mammals.

[0085] The lower frequencies (i.e. 500 Hz to 2 kHz in the seal example)
may have effects on fish with specialised hearing (such as clupeids) and
baleen whales and further research may need to be carried out to quantify
this. However, fish species with specialised hearing and most baleen
whales do not usually occur around fish farm locations and so this is not
anticipated to be a problem when the invention is used in a fish farm
environment.

Perception of Received Sound Pressure Levels

[0086] The general paradigm applied in current ADDs is that a high source
level is expected to cause physical discomfort or pain and therefore
results in an animal leaving an area. However, there are several problems
involved when operating at the upper end of the dynamic range of an
animal. FIG. 3 shows a qualitative representation of Steven's Law where
the magnitude of sensation is plotted against a magnitude of physical
stimulus for a sound. Two curves are shown; one shows the loudness of a
sound while the other shows the pain perception.

[0087] Steven's Law gives an approximate model for the general
relationship for the magnitude of sensation, ψ, and the magnitude of
a physical parameter, φ, as follows:

Ψ=k(φ-φ0)m

where k is a constant, φ0 is the lowest perceivable physical
stimulus (threshold), and m is a modality specific coefficient
determining the essential shape of the function, in the human auditory
system, m is equal to 0.6 (this value is illustrated in FIG. 3).

[0088] It can be seen that, as a generalisation, adding a defined sound
pressure value (in Pascals) to the high sound pressure stimulus leads
only to a small increase of the perceived loudness while adding the same
sound pressure value to a low sound pressure stimulus would lead to a
stronger increase in perceived loudness. Thus, an increase in sound
pressure in the upper range of the curve in FIG. 3 disproportionately
increases the risk of damaging the auditory system without yielding a
much stronger aversive effect.

[0089] The perceived loudness of a sound is generally measured on the sone
scale, a doubling of which reflects a doubling of perceived loudness. One
sone is defined as a sound that is perceived as equally loud as a 40 dB
re 20 μPa tone at 1 kHz in air for humans. The perceived loudness in
sones (L) can be calculated by the equation: L=0.01 (p-p0)-0.6,
where p is the sound pressure in μPa and p0 is the effective
threshold.

[0090] FIG. 4 shows the calculated loudness perception of a 2.5 kHz tone
in a harbour seal. A doubling of the loudness in sones reflects a
doubling in perceived loudness. The discomfort threshold for the harbour
seal is taken to lie at about 6 sones, which is slightly tower than that
for humans. Pain thresholds are much higher and usually close to SPLs
that cause immediate hearing damage. Thus, current ADDs will not cause
pain in most cases, but where pain is caused there is also likely to be
hearing damage.

[0091] In light of the potential hearing damage caused by the ADDs, the
inventors recommend that no attempt should be made to increase the source
levels of current ADDs or to use devices that emit sound continuously at
source levels at the upper end of the dynamic range close to the
suspected pain threshold. Additionally, the critical level of 135 dB
above the threshold should not be exceeded at reasonable distances from
the sound source as the risk of damage originating from single short term
exposures is substantially increased above this level.

[0092] A safe exposure level for seals would be a perceived sound exposure
level of about 126 Pas above the threshold, which equals a SEL of 183 dB
re 1 μPa2s-1. This was calculated for a 2.5 kHz tone played
to a harbour seal.

[0093] Recovery times in sound exposure scenarios that do not cause a TTS
should be at least ten seconds to avoid accumulation of acoustic trauma.
However, acceptable exposure levels should be calculated for the species
with the most sensitive hearing in the frequency range used by the ADD.
For the currently available ADDs this would usually be an odontocete
rather than a pinniped.

Types of Sounds

[0094] Killer whale sounds have been shown to be aversive to seals.
However, they equally affect cetaceans, making them an unattractive
choice. Use of predator sounds also brings the risk that mammals
habituate to the predator sounds and become more vulnerable to real
predation.

[0095] No data is available on aversiveness perception in marine mammals.
However, the hearing system is generally similar among all mammals, in
particular the basic functioning of the cochlea and peripheral auditory
processing in the brain. The inventors propose that data based on human
sound perception would be a good starting point for the investigation of
aversiveness perception in marine mammals.

[0096] Two different versions of ADDs are proposed. The first version uses
the mammalian startle response to elicit a flight. Startle sounds that
are able to induce the startle reflex response so as to provoke a flight
response and repel mammals from a location, exploit an oligo-synaptic
reflex arc related to emotional processing in the brain.

[0097] In order to elicit the startle reflex response method it is
proposed to synthesize and project stimuli with specific acoustic
properties; none of which are employed in the current state of the art.
In an embodiment, the deterrence system projects single, isolated pulses
at random intervals with a sharp onset time. In an embodiment, each
interval between isolated pulses is longer than 100 ms, and preferably
longer than 500 ms. This helps sensitize the mammal to the sound, as
explained in greater detail below. Shorter intervals may cause "pre-pulse
inhibition" of consecutive pulses. This is the observed effect that the
magnitude of the startle response is reduced by a sound stimulus which
precedes the startle sound pulse by less than 100 ms-500 ms.

[0098] In an embodiment, the amplitude of the sound exceeds the animals'
auditory threshold by more than 80 dB within 50 ms of its onset. Hence,
projected sounds may have onset/rise-times between zero and 100 ms at
sensation levels (above auditory threshold) of at least 80 dB. The
bandwidth of the sound should be as wide as possible with the desired
frequency-range and, in an embodiment, may span at least one third of an
octave. The deterrence system ensures target-specificity by choosing a
frequency band where the auditory threshold of target-species is lower
than that of non-target-species. As an example, when seals are to be
targeted a frequency range between 500 Hz and 2 kHz may be used. In
contrast when odontocetes are targeted a signal with a centre-frequency
of 10 kHz could be used. Other values would be selected if other mammals
are targeted. These depend on the hearing thresholds of the target
species.

[0099] The startle reflex response is a physiological reflex to sound
levels and has been shown to occur at specific source levels above and
hearing threshold of a particular species. It is elicited through a
relatively simple reflex and the underlying mechanisms are likely to be
shared by mammals. The startle reflex response is usually followed by a
flee response in a direction away from the source of the sound. The
startle reflex response has been well documented in rats, but mostly for
experimental purposes to study the neuronal basis of simple learning
behaviours (e.g. sensitisation and habituation), it has not been used in
practice for a mammal deterrent device, and has furthermore never been
applied to the deterrence of marine mammals in a fish farm or any other
practical environment.

[0100] The inventors' research has shown that repeated exposure to the
certain sounds leads to increased responsiveness i.e. seals become more
likely to exhibit flight responses and start avoiding the area where they
heard the sound (`sensitisation`) [Gotz, T & Janik, VM 2011, `Repeated
elicitation of the acoustic startle reflex leads to sensitisation in
subsequent avoidance behaviour and induces fear conditioning`
incorporated herein by reference, BMC Neuroscience 12: 30]. This is the
opposite of what has been found in current systems where animals get used
to the sound and avoidance responses wane. Also, where there is a strong
food motivation (known to the target mammal) in an area, a target mammal
may not necessarily be repelled by the first pulse of the startle noise,
even if startled (i.e. the food motivation overcomes the startle reflex
response). The sensitisation effect of the startle sound proposed herein
means that, if this is the case, the mammal will be repelled (and will
stay away) after a subsequent exposure to the startle sound (i.e. after
one or more subsequent pulses).

[0101] An additional benefit of the proposed startle sound is that it can
comprise short, randomly spaced pulses with a low duty cycle. Therefore
the energy consumption of the system can be very low, compared to current
ADDs, and battery life increased. Each pulse may, for example, be less
than 10 second, or less than 1 second. Similarly the duty cycle may be
less than 25%, or less than 10%.

[0102] The second version uses aversive sounds that were designed on the
basis of a model describing what makes sound pleasant or unpleasant for
humans; see Zwicker, E. & Fasti, H. (1990), Psychoacoustics--Facts and
Models, Springer-Verlag, New York. The four parameters that are mentioned
in the model that predicts what makes sounds unpleasant in humans are
high sharpness, high roughness, low tonality and high loudness. In
addition, specific frequency differences within complex sounds have been
found to be unpleasant (e.g. frequency differences/ratios that constitute
unpleasant musical intervals).

[0103] To maximise the effects caused by sharpness, higher-frequency
signals have to be used, and so sharpness is discarded as a parameter for
seal-specific ADDs. However it could be used as a parameter in an ADD for
deterring cetaceans; in particular odontocetes (toothed whales) with good
high frequency hearing.

[0104] Low tonality can be achieved using square-wave sounds as carrier
signals that do not have very tonal characteristics (for example when
compared with a pure sine wave tone).

[0105] In order to maximise roughness the carrier signal should be
frequency-modulated. A frequency modulation of between 5 Hz and 200 Hz
would be suitable for ADDs, with a potential optimum of about 70 Hz
(corresponding to the frequency modulation most effective in humans to
cause an unpleasant stimulus).

[0106] Modulation depth of the signal can be narrowband or wideband,
ranging typically between 10% and 150%. As a particular example, a 50%
modulation depth of the centre frequency of the carrier signal may be
useful.

[0107] As mentioned above, the frequency composition of complex sounds is
also a contributing factor to the aversiveness of a sound. Complex sounds
that consist of partials (individual sine wave components) having
frequency differences falling within 25% of a critical bandwidth are
perceived as dissonant in humans. When modelling the hearing system as a
series of band-pass filters, the critical bandwidth reflects the
bandwidth of each individual filter.

[0108] Some examples of suitable sounds that can be synthesised are as
follows: [0109] 1. 70-Hz frequency modulated square-wave signals with a
carrier frequency of 500 Hz and 527 Hz. Both tones are mixed (presented
at the same time) resembling the musical interval of a minor second.
Modulation depth was 50%. [0110] 2. 70-Hz frequency-modulated square-wave
tones with carrier frequencies of 500 Hz and 507 Hz. Both tones are mixed
(presented at the same time) resembling a frequency distance of 25% of
the critical band for a harbour seal (Phoca vitulina). Modulation depth
was 50%. [0111] 3. 70-Hz frequency modulated square-wave tones with a
carrier frequency of 500 Hz. [0112] 4. A combination of the first three
sounds: This sound would consist of elements of variable length (100 ms
to a few seconds). Some of these elements are presented as FM sweeps
covering a frequency range from 200 Hz to 4 kHz.

[0113] It is to be appreciated that these four examples are for
illustration purposes only,

[0114] It has also been observed that broadband signals are perceived to
be louder than narrowband signals when played at the same source level,
and this can be used to increase the perceived loudness without actually
increasing the source level. Thus, for both startle and aversive sounds,
a signal is intentionally constructed to be as broadband as possible
within the designated frequency band.

[0116] Habituation could be avoided or at least delayed by a triggering
method which only plays sounds when seals approach. This can be triggered
by the detection of a seal itself or by the analysis of changing patterns
of motion in the swimming of the fish indicating that a predator is
present. Using highly variable sound types should also prevent
habituation, but no empirical data for animals in the feeding context are
available to support this.

[0117] Studies in the startle reflex response of rats have indicated that
habituation is not caused by an increase of the perceptional threshold
eliciting the startle reflex response, but by a change of the slope of
the function of the difference between an input signal (SPL) and an
output signal (magnitude of response).

[0118] This supports the dual process theory of habituation meaning that
the response to a repeated stimulus is influenced by a decreasing
(sensitisation) and increasing (habituation) component. For ADDs this
would mean that the source levels would have to be increased beyond the
initial levels to yield the same response as before habituation occurred.
Given the abovementioned problems associated with high SPL noise, this is
not a good solution.

[0119] Ideally, one would aim to replace habituation by sensitization to a
sound stimulus. This could be achieved by using high intensity sound
intermittently to sensitize a low intensity stimulus. Sensitization
through electric stimulation is not feasible since the seal would have to
be very close to yield an effect.

[0120] In one embodiment, the acoustical stimulus is repeatedly negatively
reinforced by an aversive stimulus. Classical conditioning paradigms
could be used here. An unconditioned stimulus (e.g. startle sound)
causing an unconditioned response (e.g. startle reflex response) is
associated with a conditioning stimulus (e.g. an artificial acoustic
signal with no biological meaning) which is then able to cause the
conditioned response consisting of the same behavioural pattern as the
unconditioned response (e.g. startle and flee).

Confirmation of Theory

[0121] To confirm the theoretical improvements in acoustic deterrence
introduced above, the inventors carried out a number of experiments. To
begin with these experiments were conducted in a controlled environment
with captive seals building up to a field trial of the invention at a
fish farm in which an Acoustic Deterrent Device (ADD) of any type had
never been used.

Sensitization to Startle Sounds in Captivity

[0122] Experiments with captive seals were carried out under strong food
motivation elicited by the presence of an underwater feeding station. The
majority of the seals showed a clear startle reflex response as indicated
by neck twitches in response to the chosen stimulus (filtered noise pulse
450 Hz and 1.9 kHz; peak frequency 1 kHz, received level 170 db re 1
μPa). The startle reflex response was followed by a flight response.
After several exposures seals also started to haulout (leave the water)
in response to the sound. These flight responses and the time the animal
spent on land built up over time with all animals being very reluctant to
enter the pool or come close to the feeding station by the end of the
experiments. Aversive behaviour was quantified by an index of
aversiveness which was a cumulative index of occurrence of a series of
aversive behaviours. Depending on whether all or none of the following
behaviours occurred the index ranged from 0 (not aversive) to 4 (highly
aversive): [0123] Fish catch prevented. [0124] Seal suddenly turns away
from speaker. [0125] Escape/Flight response: seal increases distance to
speaker at speeds of more than 3 m/s. [0126] Hauling-out for at least 30
s after flight response

[0127] As shown in FIG. 7 aversive responses increased over time meaning
that animals did not habituate but sensitised to the sound. This confirms
the theory and is a highly desirable outcome for an acoustic deterrence
method. No previous study on acoustic deterrence devices has demonstrated
any kind of sensitisation to an acoustic stimulus.

Field Trial on Fish Farm

[0128] An ADD was used comprising of a Lubell® 9162 loudspeaker, a
Cadence®Z9000 stereo high-power car amplifier, a Panasonic SL-S120 CD
player and a car battery installed in a waterproof aluminium box. The
speaker was deployed at 17 m depth, which was about 2 m below the deepest
part of a cage in order to avoid sound shadow effects by the fish in the
near field. A startle stimulus comprising of a 200 ms long noise pulse
with a rise time of 5 ms and peak frequency of 950 Hz, was used. The
-0.20 dB bandwidth spanned approximately two to three octaves with the
average -20 dB power points being at 450 Hz and 1.9 kHz. The startle
pulse was paired with a substantially weaker pre-sound comprising of a 3
Hz frequency modulated 1.2 s long sine wave pure tone. The sweeps caused
by the frequency modulation covered a frequency range from 700 Hz to 1.3
kHz and the pre-sound ended 2 s before presentation of the startle pulse.

[0129] The noise pulses were played at varying intervals ranging from 2 s
to 40 s with an average of 2.4 pulses per minute. In order to make the
sound pattern less predictable the signals were arranged digitally into 4
different playback tracks each of which was 1.5 hours long. These
playback tracks were assigned to different playback days and played in
loop mode from the CD player. Given the signal length of 200 ms, the
effective duty cycle of the ADD was 0.08%. The source level of the ADD
was adjusted to 180 dB re 1 μPa. Experiments were carried out in sea
states less than 3 (slight, 0.5 to 1.25 m wave heights). Good weather
days were chosen to use as either a control day with the equipment in
place but no playbacks or an exposure day in which the stimulus was
played as described above. Average observation periods were 3.5 hours
(SD=0.96) on control days and 3.4 hours (SD=0.94) on days with sound
exposure. The longest observation period was 5 hours, the shortest 1.5
hours. This protocol resulted in a total of 113 hours of observation with
58 h during sound exposure and 55 h during control periods. Observations
were balanced so that tidal state, sea state and time of day were the
same for control and sound exposure observation periods.

[0130] During observation periods, visual scans were conducted by two
observers. One observer was scanning by eye while the other observer was
using binoculars. If one of the observers detected a porpoise or seal,
bearings and surface positions for each surfacing bout were logged. A
group was tracked until no resurfacing occurred 15 min after the last
surfacing had been logged. Group and track ID was therefore defined as a
consecutive line of surfacings that were not separated by more than 15
min. If another group or species was spotted by the observers while
tracking a group, surfacings were logged for both groups.

[0131] FIG. 5 and FIG. 6 are graphs showing a comparison of seal (FIG. 5A
and FIG. 58) and porpoise (FIG. 6A and FIG. 5B) sightings on "control"
days, on which no sounds were played, and "sound" days, on which the
sounds described above were played for (A) closest approaches and (B)
average distance from the ADD. The results show that seals clearly showed
an avoidance response up to 250 m in their closest approaches (FIG. 5A)
and in the average distance to the device (FIG. 5B) while porpoises
showed no significant differences in either response variable. Porpoise
groups were regularly seen swimming between the cages of the fish farm
and one porpoise group approached the ADD as close as 7 m during sound
exposure. The closest ever observed approach of a seal was approx 50 m
during sound exposure.

[0132] The field trial showed that an ADD incorporating "startle" sounds
did not have a negative effect on harbour porpoise distribution but was
effective in reducing the number of seals in the vicinity of the fish
farm. This fish farm was unusual in that minke whales used the sea loch
with the farm occasionally. The startle sound did not have any effects on
minke whales. In the experiments, they were exposed to received level of
up to 125 dB re 1 μPa. In fact, considerably more minke whales were
spotted on sound exposure days, as oppose to days in which no sound was
played as a control level, confirming that the animals do not show a
strong avoidance response to the ADD sounds as disclosed herein

Other Applications

[0133] Various improvement and modifications may be made to the above
without departing from the scope of the invention. In particular, while
embodiments have been described with reference to marine environments, it
is to be appreciated that the principles of the invention can be equally
applied for the deterrence of any mammal, in sea or on land. The economic
advantages applicable to the fish farm industries could be equally
applicable to other industries like game reserve control and estate
management.

Patent applications by The University Court of the University of St. Andrews